Telemetry apparatus and method for an implantable medical device

ABSTRACT

An apparatus and method for enabling radio-frequency communications with an implantable medical device utilizing far-field electromagnetic radiation. Such radio-frequency communications can take place over much greater distances than with inductively coupled antennas.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/727,093, filed on Nov. 30, 2000, the specification of whichis incorporated by reference herein.

FIELD OF THE INVENTION

[0002] This invention pertains to implantable medical devices such ascardiac pacemakers and implantable cardioverter/defibrillators. Inparticular, the invention relates to an apparatus and method forenabling radio-frequency telemetry in such devices.

BACKGROUND

[0003] Implantable medical devices, including cardiac rhythm managementdevices such as pacemakers and implantable cardioverter/defibrillators,typically have the capability to communicate data with a device calledan external programmer via a radio-frequency telemetry link. A clinicianmay use such an external programmer to program the operating parametersof an implanted medical device. For example, the pacing mode and otheroperating characteristics of a pacemaker are typically modified afterimplantation in this manner. Modern implantable devices also include thecapability for bidirectional communication so that information can betransmitted to the programmer from the implanted device. Among the datawhich may typically be telemetered from an implantable device arevarious operating parameters and physiological data, the latter eithercollected in real-time or stored from previous monitoring operations.

[0004] Telemetry systems for implantable medical devices utilizeradio-frequency energy to enable bidirectional communication between theimplantable device and an external programmer. An exemplary telemetrysystem for an external programmer and a cardiac pacemaker is describedin U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned toCardiac Pacemakers, Inc., the disclosure of which is incorporated hereinby reference. A radio-frequency carrier is modulated with digitalinformation, typically by amplitude shift keying where the presence orabsence of pulses in the signal constitute binary symbols or bits. Theexternal programmer transmits and receives the radio signal with anantenna incorporated into a wand which can be positioned in proximity tothe implanted device. The implantable device also generates and receivesthe radio signal by means of an antenna, typically formed by a wire coilwrapped around the periphery of the inside of the device casing.

[0005] In previous telemetry systems, the implantable device and theexternal programmer communicate by generating and sensing a modulatedelectromagnetic field in the near-field region with the antennas of therespective devices inductively coupled together. The wand must thereforebe in close proximity to the implantable device, typically within a fewinches, in order for communications to take place. This requirement isan inconvenience for a clinician and limits the situations in whichtelemetry can take place.

SUMMARY OF THE INVENTION

[0006] The present invention is an apparatus and method for enablingcommunications with an implantable medical device utilizing far-fieldelectromagnetic radiation. Using far-field radiation allowscommunications over much greater distances than with inductively coupledantennas. In accordance with the invention, a conductor extending fromthe implantable device acts as an antenna that radiates and receivesfar-field radio-frequency radiation modulated with telemetry data. Theantenna is dimensioned such that a substantial portion of theradio-frequency energy delivered to it at a specified frequency by atransmitter in the implantable device is emitted as far-fieldelectromagnetic radiation. A tuning circuit may be used to tune theantenna by optimizing its impedance. In one embodiment, a therapy leadof a cardiac rhythm management device which is otherwise used forstimulating and/or sensing electrical activity in the heart hasincorporated therein a wire antenna. Specialized structures may also beincorporated into such a therapy lead in order to isolate a separateantenna section therein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 shows an implantable medical device with an antennaextending from the device.

[0008]FIG. 2 shows a bipolar therapy lead.

[0009]FIG. 3 is a block diagram of a cardiac rhythm management deviceutilizing a therapy lead as an antenna.

[0010]FIG. 4 shows a therapy lead with a separate antenna section.

DETAILED DESCRIPTION

[0011] As noted above, conventional radio-frequency (RF) telemetrysystems used for implantable medical devices such as cardiac pacemakersutilize inductive coupling between the antennas of the implantabledevice and an external programmer in order to transmit and receive RFsignals. Because the induction field produced by a transmitting antennafalls off rapidly with distance, such systems require close proximitybetween the implantable device and a wand antenna of the externalprogrammer in order to work properly, usually on the order of a fewinches. The present invention, on the other hand, is an apparatus andmethod for enabling telemetry with an implantable medical deviceutilizing far-field radiation. Communication using far-field radiationcan take place over much greater distances which makes it moreconvenient to use an external programmer. Also, the increasedcommunication range makes possible other applications of the telemetrysystem such as remote monitoring of patients and communication withother types of external devices.

[0012] A time-varying electrical current flowing in an antenna producesa corresponding electromagnetic field configuration that propagatesthrough space in the form of electromagnetic waves. The total fieldconfiguration produced by an antenna can be decomposed into a far-fieldcomponent, where the magnitudes of the electric and magnetic fields varyinversely with distance from the antenna, and a near-field componentwith field magnitudes varying inversely with higher powers of thedistance. The field configuration in the immediate vicinity of theantenna is primarily due to the near-field component, also known as theinduction field, while the field configuration at greater distances isdue solely to the far-field component, also known as the radiationfield. The near-field is a reactive field in which energy is stored andretrieved but results in no net energy outflow from the antenna unless aload is present in the field, coupled either inductively or capacitivelyto the antenna. The far-field, on the other hand, is a radiating fieldthat carries energy away from the antenna regardless of the presence ofa load in the field. This energy loss appears to a circuit driving theantenna as a resistive impedance which is known as the radiationresistance. If the frequency of the RF energy used to drive an antennais such that the wavelength of electromagnetic waves propagating thereinis much greater than the length of the antenna, a negligible far-fieldcomponent is produced. In order for a substantial portion of the energydelivered to the antenna to be emitted as far-field radiation, thewavelength of the driving signal should not be very much larger than thelength of the antenna.

[0013] An antenna most efficiently radiates energy if the length of theantenna is an integral number of half-wavelengths of the driving signal.A dipole antenna, for example, is a center-driven conductor which has alength equal to half the wavelength of the driving signal. A shorterantenna can produce a similar field configuration by utilizing a groundplane to reflect electromagnetic waves emitted by the antenna andthereby produce an image field. A monopole antenna is a conductor with alength equal to one-quarter the wavelength of the driving signalsituated with respect to a reflecting ground plane so that the totalemitted and reflected field configuration resembles that of the dipoleantenna. As will be discussed below, an antenna tuning circuit may beused to alter the effective electrical length of an antenna by loadingit with capacitance or inductance.

[0014]FIG. 1 shows an exemplary implantable medical device 100 with anantenna 110 suitable for radiating and receiving far-fieldelectromagnetic radiation extending from the device housing 101. Thedevice housing 101 contains electronic circuitry for providingparticular functionality to the device such as cardiac rhythmmanagement, physiological monitoring, drug delivery, or neuromuscularstimulation as well as providing RF communications. The antenna 110includes a conductor covered by insulation that is electricallyconnected to a radio-frequency transmitter/receiver within the housing.In various embodiments, the antenna 110 may be any conductive structurecapable of efficiently radiating electromagnetic energy well-known tothose of skill in the art such as a rod, a wire, a patch, or a loop. TheRF circuitry transmits and receives a carrier signal at a specifiedfrequency that is modulated with telemetry data.

[0015] In order for a substantial portion of the RF energy delivered tothe antenna 110 to be emitted as far-field radiation, the length of theantenna should not be very much shorter than one-quarter of thewavelength of the RF carrier signal provided by the RF transmitter. Inan exemplary embodiment, the carrier signal is selected to be 1gigahertz which corresponds to a wavelength of approximately 32 cm. Aquarter-wavelength monopole antenna can be formed by a wire antennahaving a length approximately 8 cm with the housing 101 being made ofmetal and serving as a ground plane. The device 100 can then beimplanted in an appropriate body location with the antenna 110 extendingfrom the device housing.

[0016] In another embodiment of the invention, an antenna may beincorporated into a therapy lead of a cardiac rhythm management device.Such leads are designed to be disposed intravascularly and serve toconnect rhythm control circuitry within the device housing to electrodesthat deliver stimulation and sense cardiac activity in the heart.Cardiac rhythm management devices, which include pacemakers andimplantable cardioverter/defibrillators, are battery-powered implantabledevices with rhythm control circuitry for sensing cardiac activity andelectrically stimulating the heart by means of one or more electrodes inelectrical contact with the myocardium. Such stimulation is used eitherto pace the heart or to terminate arrhythmias such as ventricularfibrillation. A cardiac rhythm management device is usually implantedsubcutaneously on the patient's chest, and a therapy lead threadedthrough the vessels of the upper venous system into the heart connectsthe rhythm control circuitry of the device to an electrode or electrodepair. Typically, such therapy leads are either unipolar or bipolar withone or two electrodes, respectively, at the distal end of the lead whichcan be used for either sensing or stimulation of the heart. The leadincludes a conductor for each electrode surrounded by an insulatingcovering, with the conductor or conductors constituting a wire antennain addition to electrically connecting the sensing/stimulation electrodeto circuitry within the device housing. Such a therapy lead, or asimilar structure, may also be used as a dedicated antenna.

[0017]FIG. 2 is a cross-sectional view of a bipolar therapy lead 200showing conductors 201 and 202 that connect to a ring electrode 211 andtip electrode 212, respectively. The conductors 201 and 202 are normallyused to convey stimulation pulses and sensing signals to and from theelectrodes. The conductors are insulated from one another within thelead and may be coiled so as to impart flexibility to the lead. Theconductors can also be driven with RF energy, however, and serve as anantenna for transmitting and receiving RF signals similar to thatdescribed above with reference to FIG. 1. Incorporating an antenna intoan intravasculary disposed therapy lead allows the antenna to be longerthan an antenna confined to the proximity of the housing in itsimplanted location and thus permits the use of lower carrierfrequencies. For example, a carrier frequency of 403 megahertzcorresponds to a wavelength of approximately 74 cm. A quarter-wavelengthmonopole antenna may then be formed by a therapy lead with an antenna ofabout 18 cm in length, which length can easily be accommodated withinthe vasculature.

[0018]FIG. 3 is a block diagram of a cardiac rhythm management device inwhich a therapy lead 310 is used as an RF antenna. The device includes ametallic housing (shown as housing 101 in FIG. 1 and typically made oftitanium) with feedthroughs for enabling the therapy leads to connect tocomponents internal to the housing. In the figure, only one therapy lead310 is shown but it should be understood that a cardiac rhythmmanagement device may use two or more such leads. A microprocessorcontroller 302 controls the operation of the rhythm control circuitry320. Rhythm control circuitry 320 includes sensing and stimulusgeneration circuitry that are connected to electrodes by the therapyleads. The conductors of the therapy lead 310 connect to rhythm controlcircuitry 320 through an RF choke filter 321 that serves to isolate thecircuitry 320 from RF signals that are received by the antenna/lead 310or are transmitted to the antenna by RF drive circuitry 330. The RFdrive circuitry 330 includes an RF transmitter and receiver that areconnected by a transmit/receive switch 333 to the antenna. Themicroprocessor 302 outputs and receives the data contained in themodulated carrier generated or received by the drive circuitry 330.

[0019] In this embodiment, the RF drive circuitry 330 is connected tothe antenna/lead 310 through an antenna tuning circuit which loads theantenna/lead 310 with a variable amount of inductance or capacitance tothereby adjust the effective electrical length of the antenna and matchthe antenna impedance to the impedance of the transmitter/receiver. Inthis manner, the reactance of the antenna may be tuned out so that theantenna forms a resonant structure at the specified carrier frequencyand efficiently transmits/receives far-field radiation. The tuningcircuit in this embodiment is a tank circuit made up of an inductor 348and a capacitor 350. A variable amount of capacitance is added to thetank circuit by a varactor diode 341 that can be controlled by a tuningbias voltage provided by a digital-to-analog converter 342. An RF chokefilter 344 isolates the digital-to-analog converter 342 from the RFcircuitry while allowing it to set the DC voltage of the varactor diode341. A DC blocking capacitor 346 isolates the RF circuitry from the DCvoltage across the varactor diode. By adjusting the voltage of thevaractor diode 341, the antenna can be tuned to various carrierfrequencies under control of the microprocessor. This makes it possibleto use various antenna structures of different dimensions at a specifiedcarrier frequency as well as to efficiently radiate energy at a widerange of frequencies. Examples of antenna structures with which thetuning circuit can be used include antennas disposed within anon-conductive portion of the housing and patch antennas mounted on thehousing, as well as lead or other antennas extending from the housing asdescribed herein.

[0020] When a therapy lead is used as an antenna as described above, RFenergy may also be delivered to the heart through the electrodes of thelead when the antenna is transmitting. This energy is minimal, however,since at 403 MHz and at 25 microwatts of transmitted power, the voltageinduced on an electrode of a therapy lead will only be about 1 or 2millivolts during transmission. (When the antenna is receiving an RFsignal at that power level and frequency, the induced voltage on theelectrode will be much less, on the order of a few microvolts.) This iswell below the pacing threshold and should not interfere with theintrinsic activity of the heart. Also, the frequency of the RF signal issuch that the duration of voltage pulses at the lead electrode is tooshort to stimulate the heart. To further ensure that an arrhythmia willnot be induced by an RF transmission, however, telemetry bursts from theimplanted device can be made synchronous with detected intrinsicactivity or with pacing pulses.

[0021] Although standard therapy leads in use today can be used as afar-field antenna as described above, a therapy lead can also bedesigned with specific features for antenna use. FIG. 4 shows such atherapy lead 400 in longitudinal cross-section. A transversecross-sectional view of three separate-sections of the lead, designated400 a through 400 c, is also shown. The lead is a bipolar lead withconductors 401 and 402 connected to ring and tip electrodes 411 and 412,respectively, located at the distal end of the lead. At the proximal endof the lead is a connector assembly 413 for connecting to the cardiacrhythm management device. The conductors within the lead constitute atransmission line, the characteristics of which are determined by theirgeometrical arrangement. For example, helically coiling the conductorincreases the inductance of the line and decreasing the distance betweenthe conductors increases the capacitance. In a distal section 400 c ofthe lead shown in FIG. 4, the conductors 401 and 402 may be disposedclose together to result in enough parasitic capacitance between the tipand ring electrodes to form a low-pass filter that effectively removesany RF frequencies from the signal transmitted to or from theelectrodes. Inductance can also be added to section by coiling theconductors to form resonant trap or notch filter that removes aspecified RF frequency band. In the middle section 400 b of the lead,the conductors 401 and 402 are arranged in parallel and at a sufficientdistance apart to radiate electromagnetic energy and thereby form aradiating section. The radiating section of the lead can be made to beone-quarter wavelength of the RF carrier frequency and used with aground plane to efficiently produce far-field radiation. In order tofacilitate the placement of the lead in the body without affecting thedesired one-quarter wavelength of the radiating section, a proximalsection 400 a is provided in which the conductors are geometricallyarranged so as to be relatively non-radiating. In a particularembodiment, one of the conductors 401 or 402 is helically wound aroundthe other so as to serve as shielding similar to a coaxial cable withline currents confined to the inner surface of the outer conductor. Thesection 400 a thus constitutes a transmission-line section that conveysRF energy to the radiating section 400 b without affecting the fieldconfiguration. This allows the section 400 a to be coiled or otherwisearranged by the physician during lead implantation without affecting theradiating part of the antenna.

[0022] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A method for transmitting and receivingradio-frequency signals in an implantable cardiac rhythm managementdevice, comprising: sensing intrinsic cardiac electrical activity froman intravascular therapy lead; transmitting or receiving a modulatedradio-frequency carrier at a specified frequency to or from an antennaincorporated into the therapy lead; and, transmitting the modulatedradio-frequency carrier in the form of telemetry bursts made synchronouswith detected intrinsic cardiac electrical activity.
 2. The method ofclaim 1 further comprising; delivering cardiac pacing pulses through thetherapy lead; and, transmitting the modulated radio-frequency carrier inthe form of telemetry bursts made synchronous with cardiac pacingpulses.
 3. The method of claim 1 further comprising emitting asignificant portion of radio-frequency energy delivered to the antennaat the specified frequency as far-field radiation.
 4. The method ofclaim 1 wherein the wavelength of the radio-frequency carrier isapproximately four times or less the electrical length of the antenna.5. The method of claim 1 further comprising adjusting the electricallength of the antenna by loading the antenna with inductance orcapacitance using a tuning circuit.
 6. The method of claim 1 furthercomprising transmitting the radio-frequency signal at a wavelength thatcorresponds to a resonant frequency of the antenna.
 7. The method ofclaim 1 further comprising employing the housing as a ground plane forthe antenna.
 8. A method for transmitting and receiving radio-frequencysignals in an implantable cardiac rhythm management device, comprising:delivering cardiac pacing pulses through an intravascular therapy lead;and, transmitting or receiving a modulated radio-frequency carrier at aspecified frequency to or from an antenna incorporated into the therapylead; and, transmitting the modulated radio-frequency carrier in theform of telemetry bursts made synchronous with cardiac pacing pulses. 9.The method of claim 8 further comprising emitting a significant portionof radio-frequency energy delivered to the antenna at the specifiedfrequency as far-field radiation.
 10. The method of claim 8 wherein thewavelength of the radio-frequency carrier is approximately four times orless the electrical length of the antenna.
 11. An implantable cardiacrhythm management device, comprising: a housing for containingelectronic circuitry; rhythm control circuitry electrically connected byone or more conductors within a therapy lead to an electrode adapted fordisposition within the heart; an antenna extending from the housing,wherein the antenna is formed by the conductors of the therapy lead;circuitry within the housing for transmitting or receiving a modulatedradio-frequency carrier at a specified frequency to or from the antenna,wherein the circuitry transmits the modulated radio-frequency carrier inthe form of telemetry bursts made synchronous with detected intrinsiccardiac electrical activity from the therapy lead.
 12. The device ofclaim 11 wherein the circuitry transmits the modulated radio-frequencycarrier in the form of telemetry bursts made synchronous with cardiacpacing pulses delivered through the therapy lead.
 13. The device ofclaim 11 further comprising an antenna tuning circuit for adjusting theelectrical length of the antenna by loading the antenna with inductanceor capacitance.
 14. The device of claim 11 wherein the electrical lengthof the antenna is such that a significant portion of radio-frequencyenergy delivered to the antenna at the specified frequency is emitted asfar-field radiation.
 15. The device of claim 11 wherein the electricallength of the antenna is approximately one-quarter or greater of thewavelength of the radio-frequency carrier at the specified frequency.16. The device of claim 11 wherein the housing serves as a ground planefor the antenna.
 17. The device of claim 11 wherein the conductorswithin a proximal portion of the therapy lead are arranged so as to forma non-radiating transmission line section of the lead that can be coiledwithout affecting a radiating antenna section of the lead.
 18. Thedevice of claim 17 wherein one of the conductors within the proximalportion of the therapy lead is helically wound around the otherconductor.
 19. The device of claim 11 wherein the conductors within adistal portion of the lead adjacent to the electrode are arranged toform a filter section that blocks the transmission of radio-frequencyenergy to or from the electrode.
 20. The device of claim 11 wherein theconductors within a distal portion of the lead adjacent to the electrodeare arranged to form a notch filter section that blocks the transmissionof radio-frequency energy at a specified frequency band to or from theelectrode.